Backlight module is a key element of the actuating light source of a display panel. Besides, providing a lighting source, the dimming function to alter the actual light projection effect in response to the environment illuminating condition is the basic function of the backlight module in practical applications.
The actuating electric source for the backlight modules now on the market mostly adopts high voltage inverters. They can be classified in current feeding push-pull parallel resonant inverters and single stage inverters. The transformers used in the inverters include winding transformers and piezoelectric transformers. Their duty cycle waveforms are shown in FIGS. 1A and 2A (FIG. 1A is a burst mode dimming method, FIG. 2A is a standby mode dimming method, technical details can be found in U.S. Pat. No. 6,839,253). For discussion purpose, assuming input voltage is DC 10V, the dimming efficiency (dimming duty cycles are 1a and 1c) is 100%, then the electric conductive interval of the electric conductive cycles 2a and 2c is 50% ON and 50% OFF. The transformer oscillation duty cycles 3a and 3c are 100% sinuous waveform based on the amplitude of 10V. Assuming the load (cold cathode lamp) outputs a lamp feedback current of 6 mA, when the input voltage is altered to 20V, as shown in FIGS. 1B and 2B, the existing dimming mechanism relatively increases the lamp feedback current (such as 12 mA) when the input voltage alters. In order for the backlight module to maintain the illumination at the existing dimming efficiency, the lamp current feeds back electricity to the dimming controller (or getting a voltage feedback electricity from the transformer output end or input end as the comparison value). By comparing the feedback electricity with a reference value built in the dimming controller, a second dimming duty cycle is determined. As shown in the drawings, when the input voltage is altered to DC of 20V, the dimming duty cycles 1b and 1d are transformed to 50% ON and 50% OFF. The electric conductive cycles 2b and 2d are changed to 50% ON and 50% OFF when the electric conductive interval is maintained 50% ON and 50% OFF. The transformer oscillation duty cycles 3a and 3d are 50% ON and 50% OFF at the amplitude of 20V. Referring to FIGS. 3A and 4A, the input voltage is DC 10V, the dimming efficiency (dimming duty cycles 1e and 1g) is 50%. FIGS. 3B and 4B show that the input voltage is DC 20V, the dimming efficiency (dimming duty cycles 1f and 1h) is changed to 25%. The assumed conditions for the rest electric conductive cycles 2e, 2g, 2f and 2h, and the oscillation duty cycles 3e, 3g, 3f and 3h are same as previously discussed. While such a dimming control mechanism can maintain the existing efficiency and illumination for the cold cathode lamp, as shown in the drawings, there are still drawbacks, notably:
1. As the dimming duty cycle is squeezed, the actual applicable dimming range of the backlight module is affected. As shown in FIG. 3B, the applicable dimming range of the existing backlight module mostly is between 20% and 100%. But when the input voltage has great alterations, the transformer that originally has 50% of dimming efficiency could result in a single sinuous waveform at 25%. As a result, the backlight module cannot continuously correct the dimming efficiency downwards, Hence the actual dimming efficiency range is limited between 50% and 100%, not the original setting of 20% to 100%.
2. The oscillation amplitude of the transformer and actuator is changed (such as from 10V to 20V) when the input voltage is altered. This will shorten the life span of the transformer and actuator.
3. When the input voltage fluctuates greatly and is not stable, the lamp current of the cold cathode lamp will generate a higher waveform factor. As a result, blacking phenomenon is easily occurred to the ignition end of the cold cathode lamp.